macos - Why Swift is 100 times slower than C in this image processing test?

Like many other developers I have been very excited about the new
Swift language from Apple. Apple has claimed its speed is faster than Objective C and
can be used to write operating system. And from what I learned so far, it's a static
typed language and able to have precisely control over the exact data type (like integer
length). So it does look like having good potential handling performance critical tasks,
like image processing, right?

That's what I
thought before I carried out a quick test. The result really surprised me.

Here is a simple code snippet in
C:

test.c:

#include
            
#include 
#include
            

uint8_t pixels[640*480];
uint8_t
            alpha[640*480];

uint8_t blended[640*480];

void
            blend(uint8_t* px, uint8_t* al, uint8_t* result, int size)
{

            for(int i=0; i result[i] = (uint8_t)(((uint16_t)px[i]) *al[i]
            /255);
 }
}

int
            main(void)

{
 memset(pixels, 128, 640*480);

            memset(alpha, 128, 640*480);
 memset(blended, 255,
            640*480);

 // Test 10 frames
 for(int i=0; i<10; i++)
            {
 blend(pixels, alpha, blended, 640*480);

            }


 return
            0;
}

I
compiled it on my Macbook Air 2011 with the following
command:

clang -O3 test.c -o
            test

The 10 frame
processing time is about 0.01s. In other words, it takes the C code 1ms to process one
frame:

$ time
            ./test
real 0m0.010s
user 0m0.006s
sys
            0m0.003s

Then I have a
Swift version of the same
code:

test.swift:

let
            pixels = UInt8[](count: 640*480, repeatedValue: 128)
let alpha =
            UInt8[](count: 640*480, repeatedValue: 128)
let blended = UInt8[](count:
            640*480, repeatedValue: 255)

func blend(px: UInt8[], al: UInt8[],
            result: UInt8[], size: Int)
{
 for(var i=0; i            {
 var b = (UInt16)(px[i]) * (UInt16)(al[i])
 result[i] =
            (UInt8)(b/255)

 }
}

for i in 0..10
            {
 blend(pixels, alpha, blended,
            640*480)
}

The
build command line
is:

xcrun swift -O3
            test.swift -o
            test

Here I use the
same O3 level optimization flag to make the comparison
hopefully fair. However, the resulting speed is 100 time
slower:

$ time
            ./test

real 0m1.172s
user 0m1.146s
sys
            0m0.006s

In
other words, it takes Swift ~120ms to processing one frame which takes C just 1 ms.

What
happened?

Update: I am using
clang:

$ gcc
            -v
Configured with:
            --prefix=/Applications/Xcode6-Beta.app/Contents/Developer/usr
            --with-gxx-include-dir=/usr/include/c++/4.2.1

Apple LLVM version
            6.0 (clang-600.0.34.4) (based on LLVM 3.5svn)
Target:
            x86_64-apple-darwin13.2.0
Thread model:
            posix

Update: more
results with different running iterations:

Here are the result for different number of
"frames", i.e. change the main for loop number from 10 to other
numbers. Note now I am getting even faster C code time (cache hot?), while the Swift
time doesn't change too much:

 C
            Time (s) Swift Time (s)

 1 frame: 0.005 0.130
 10
            frames(*): 0.006 1.196
 20 frames: 0.008 2.397
100 frames: 0.024
            11.668

Update:
`-Ofast` helps

With
-Ofast suggested by @mweathers, the Swift speed goes up to
reasonable range.

On my laptop the
Swift version with -Ofast gets 0.013s for 10 frames and 0.048s
for 100 frames, close to half of the C performance.

Answer

Building
with:

xcrun swift -Ofast test.swift
-o test

I'm getting times
of:

real 0m0.052s
user
            0m0.009s

sys
            0m0.005s